Abstract: In this paper, we characterize the effect of defect kinetics on the stability of ordered nickel-based binary alloys, NiFe, NiCo and NiCu, using the kinetic Activation-Relaxation Technique (k-ART), an unbiased off-lattice kinetic Monte Carlo method with on-the-fly catalog building, that can provide kinetic pathways over second scales taking full account of chemical and elastic effects. We generate the full energy landscape surrounding vacancy and self-interstitial diffusion for L10 NiFe, an alloy with promising magnetic properties, with those of model L10 NiCo and NiCu, and combine this information with unbiased long-time kinetic simulations to characterize the link between specific microscopic diffusion mechanisms and overall phase stability. Our simulations demonstrate an unexpected richness and diversity: even though these alloys display similar proprieties like atomic radius, single vacancy and interstitial diffuse along totally different pathways that explain the relative stability of ordered structure.

Abstract: Abstract
Point defects play a central role in materials properties. Yet, details regarding their diffusion and aggregation are still largely lacking beyond the monomer and dimer. Using the kinetic Activation Relaxation Technique (k-ART), a recently proposed off-lattice kinetic Monte Carlo method, the energy landscape, kinetics and diffusion mechanisms of point defect in fcc nickel are characterized, providing an exhaustive picture of the motion of one to five vacancies and self-interstitials in this system. Starting with a comparison of the prediction of four empirical potentials — the embedded atom method (EAM), the original modified embedded atom method (MEAM1NN), the second nearest neighbor modified embedded atom method (MEAM2NN) and the Reactive Force Field (ReaxFF) —, it is shown that while both EAM and ReaxFF capture the right physics, EAM provides the overall best agreement with ab initio and molecular dynamics simulations and available experiments both for vacancies and interstitial defect energetics and kinetics. Extensive k-ART simulations using this potential provide complete details of the energy landscape associated with these defects, demonstrated a complex set of mechanisms available to both vacancies and self-interstitials even in a simple environment such as crystalline Ni. We find, in particular, that the diffusion barriers of both vacancies and interstitials do not change monotonically with the cluster size and that some clusters of vacancies diffuse more easily than single ones. As self-interstitial clusters grow, moreover, we show that the fast diffusion takes place from excited states but ground states can act as pinning centers, contrary to what could be expected.